Observed Changes of Cryosphere in China Over the Second Half of the 20Th Century: an Overview

382 Annals of Glaciology 46 2007

Observed changes of cryosphere in China over the second half of the 20th century: an overview

XIAO Cunde,1,2 LIU Shiyin,1 ZHAO Lin,1 WU Qingbai,1 LI Peiji,1 LIU Chunzhen,3 ZHANG Qiwen,4 DING Yongjian,2 YAO Tandong,1,5 LI Zhongqin,2 PU Jiancheng1 1State Key Laboratory of Cryospheric Science, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China E-mail: [email protected] 2China Meteorological Administration, 46 Zhongguancun South Avenue, Beijing 100081, China 3Water Resources Information Center, Ministry of Water Resources of China, Beijing 100053, China 4The National Center of Marine Environmental Forecast, Beijing 100081, China 5Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China

ABSTRACT. During approximately the past five decades, changes in snow cover, mountain glaciers, frozen ground (including permafrost), sea ice and river ice have been observed in China. However, most data were published in Chinese and thus unknown to the international communities. Here we review these published results to show an overview of cryospheric changes in China for the last 50 years. Long-term variability of snow cover over the Qinghai–Xizang (Tibetan) Plateau (QXP) is characterized by large interannual variability superimposed on a continuously increasing trend. Glacier changes in western China vary remarkably in different regions. Although in most mountains the glaciers display a retreating trend (80%) or have even vanished, some glaciers (20%) are still advancing. Frozen ground (including permafrost) has experienced a rapid decay since the 1980s, and these changes are occurring both in the QXP and in the cold regions of north China. Sea-ice areas in the Bohai and north Yellow Seas have been shrinking since the 1970s. Interannual variations possibly relate to the solar cyles, and sea-ice extent extremes relate to El Nin˜o–Southern Oscillation (ENSO) events. The freeze-up and break-up dates of river ice in north China in the 1990s are, on average, 1–6 days later and 1–3 days earlier, respectively, than the 1950s–1990 mean. Frozen duration and the maximum thickness of river ice are, respectively, 4–10 days shorter and 0.06–0.21 cm thinner in the 1990s than the average.

INTRODUCTION maximum area in late January to early February and melts away in late March. River ice forms in most rivers in north The cryosphere, portions of the Earth’s surface where water China, including the Yellow, Songhuajiang, Nengjiang and is in solid form, showed a widespread shrinkage in the 20th Liaohe rivers. century, the warmest century for the past 2000 years There are few reliable observations available for the (Houghton and others, 2001). The cryosphere is considered cryosphere before 1949, when the People’s Republic of as one of the sensitive indicators of climate change, and in China was established, but since then glaciers and frozen China, as one of the largest cryospheric areas on Earth, the ground over the QXP and Tien Shan have been widely cryosphere has been undergoing rapid changes in recent investigated. Since 1951, snow cover has been measured at decades. Reviewing these changes will help provide a better meteorological station networks governed by the China understanding of the present and future state of the global Meteorological Administration. Sea- and river-ice obser- cryosphere. vations were also started in the 1950s. These observations The cryosphere in China is mainly located in the give an insight into regional climate change over the past Qinghai–Xizang (Tibetan) Plateau (hereafter QXP), the half-century. However, most of the data were published in eastern Tien Shan, the Altai Shan, the east Pamirs and Chinese, and thus unknown to international communities. northeast China. The total number of glaciers in China is Presently, the Chinese National Committee of the World 46 298, with a total area of 59 406 km2 and an ice volume of 3 Climate Research Program (WCRP)/Climate and Cryo- 5590 km . The permafrost and the seasonally frozen ground sphere Project (CliC) (CliC-CNC) is working on synthesiz- 6 6 2 (SFG) cover an area of 1.49 10 and 5.28 10 km , ing cryospheric data. Here, we review these results, based accounting for 11.5% and 55% of Chinese land territory, mostly on direct observations and partly on remote-sensing respectively. The snow-covered area in China is around methods, which can certainly contribute to an overall 6 2 6 2 9.0 10 km , and of this an area over 4.8 10 km assessment of the present and future state of the global consists of unstable snow cover (duration <20 days). Stable cryosphere. snow cover (duration >60 days) is mainly located in the QXP, north Xinjiang (including the Tien Shan) and the Inner Mongolia–northeast China (hereafter IM-NEC) regions. The DATA AND SOURCES mean snow-cover areas of these three regions are 2.3 106, 0.5 106 and 1.4 106 km2, respectively. Seasonal sea ice Data on snow cover forms in the Bohai Sea and in the northern Yellow Sea of The study area extends over the latitude–longitude domain China. Sea ice forms in mid- to late November, reaches its from approximately 708 E to 1058 E and from 278 Nto508 N. Xiao and others: Observed changes of cryosphere in China 383

Table 1. Ground–based observations of glacier length in China for the past decades

Glacier (mountain range) Location Years with data available Source Profile in Fig. 2

Dongkemadi (Tanggula) 338060 N, 928050 E 1969, 1990, 1992, 1994, 2000 Pu and others (2004) Fig. 2a (2) July 1st (Qiyi) (Qilian) 398140 N, 978450 E 1956, 1984 Liu and others (2000) Fig. 2a (2) Laohugou 12 (Qilian) 398260 N, 968320 E 1962, 1960, 1985 Liu and others (2000) Fig. 2a (3) Puruogangri 6 (Puruogangri) 338500 N, 898000 E 1974, 1999, 2000 Pu and others (2004) Fig. 2a (4) 0 0 U¨ ru¨mqi No. 1 (Tien Shan) 43800 N, 86830 E 1962, 1973, 1980, 1981, 1982, Li and others (2003) Fig. 2a (5) 1986, 1990, 2000 Kangwure (Xixiabangma) 28827’ N, 85850’ E 1976, 1991, 1994, 2000 Su and Pu (1998) Fig. 2a (6) East Rongbuk (Qomolangma) 278590 N, 868550 E 1976, 1997, 1999, 2002, 2004 Ren and others (2006) Fig. 2a (7) Rongbuk (Qomolangma) 278590 N, 86855’ E 1966, 1997, 1999, 2002, 2004 Ren and others (2006) Fig. 2a (8) Lanong (Nyainqeˆntanglha) 308260 N, 90833’ E 1970, 1999, 2003 Zhang and others (2004) Fig. 2a (9) Hailuogou (Gongga) 298350 N, 1018560 E 1966, 1981, 1989, 1994, 1998 Pu and others (2004) Fig. 2a (20) Shuiguanhe 4 (Qilian) 1956, 1976, 1984 Liu and others (2000) Fig. 2a (22) Musita (Muztag) 388170 N, 758040 E 1976, 2000 Shangguan and others (2004) Fig. 2b (2) 5K451F33 (Tanggula) 1966, 2000 Lu and others (2005) Fig. 2b (2) Muqigan (Muztag) 388170 N, 758040 E 1964, 2001 Shangguan and others (2004) Fig. 2b (3) Yehelong (A’nyeˆmaqeˆn) 348300 N, 1008000 E 1966, 2000 Liu and others (2002) Fig. 2b (4) Jicongpu (Xixiabangma) 288010 N, 858470 E 1977, 1984, 1990, 1996, 2000, Che and others (2005) Fig. 2b (5) 2003 Reqiang (Xixiabangma) 288010 N, 858470 E 1977, 1984, 1990, 1996, 2000, Che and others (2005) Fig. 2b (6) 2003 Xilantai (Tien Shan) 1942, 1976, 1997 Liu and others (2000) Fig. 2b (7) 5J351D20 (A’nyeˆmaqeˆn) 348300 N, 1008000 E 1966, 2000 Lu and others (2005) Fig. 2c (2) 5J351D26 (A’nyeˆmaqeˆn) 348300 N, 1008000 E 1966, 2000 Lu and others (2005) Fig. 2c (2) Ganmoxin (Tanggula) 1969, 2000 Pu and others (2004) Fig. 2c (3) Qiangyong east (Nyainqeˆntanglha) 1975, 1979, 1980, 2000 Pu and others (2004) Fig. 2c (4) Baishuihe No. 1 (Yulong) 278300 N, 1008150 E 1957, 1982, 1998, 2002 Pu and others (2004) Fig. 2c (5) Nailuogeru (Meili) 1959, 1971, 1982, 1998, 2002 He and others (2003) Fig. 2c (6) Pu and others (2004) Xinqingfeng west (Kunlun) 368100 N, 918300 E 1971, 1976, 1987, 1994, 2000 Liu and others (2004) Fig. 2c (7) 5K451F12 (Tanggula) 1966, 2000 Lu and others (2005) Fig. 2c (8) 5K451F1 (Tanggula) 1966, 2000 Lu and others (2005) Fig. 2c (9)

Western China is physiographically divided into two re- Region. Snow-cover data from 46 stations in Xinjiang and gions: the QXP and northwestern China. The QXP, with an 64 stations in IM-NEC are used to study the long-term trend. average elevation exceeding 4500 m and an area of >2 106 km2, has been acclaimed as the ‘Roof of the Data on glaciers World’. The Himalaya, the world’s highest mountain range, Records of directly measured mass balance are obtained at a provides a natural screen on the southern frontier of the few logistically ‘easy’ glaciers: U¨ ru¨mqi glacier No. 1 (UG1), plateau. Northwestern China is an arid region surrounded by Tien Shan; Xiao Dongkemadi (XDKMD), Tanggula Shan; high mountains and large basins, such as the Altai Shan, Meikuang (MK), east Kunlun Shan; July 1st (Qiyi) glacier, Tien Shan, Pamirs, Karakoram, the Kunlun Shan and the Qilian Shan; and Hailuogou glacier, Gongga Shan. The Tarim and Junggar basins. mass-balance data series differ in length, and are discontin- The meteorological network of western China consists of uous except for UG1. more than 200 synoptic stations. It was only after 1956 that Glacier changes are defined as changes of ice mass and the QXP network became dense enough to ensure adequate glacier size, including thickness, length and width. Ground spatial coverage, except for western Tibet. To minimize the monitoring of glacier changes in China for recent decades defects of station data, a subset of the network was created has been carried out at several sites. Continuous obser- in northwestern China and the QXP, respectively. The station vations are operated only on UG1, which is the only selection criteria included availability of the longest time glacier in China that is included in the World Glacier series; few missing records; and without site relocation. The Monitoring Service (WGMS; International Satellite Com- station records were used without further adjustment, and no munication Systems–International Association of Hydro- attempt was made to fill in the few missing data (Balling and logical Sciences) network. Other ground-based Idso, 2002). Following strict quality control, the station point observations have been operated sporadically on glaciers records were integrated over the snow-cover years (Septem- covering all of the mountain ranges in western China ber to August) and space to derive regional time series. With (Table 1). regard to large-scale area averages, biases and errors For the statistical estimations of glacial changes over associated with specific point data were further minimized specific regions, studies are based on the 1950–70 topo- to such an extent that a homogeneous and meaningful signal graphic map and on Landsat Thematic Mapper (TM)/ could be extracted. Over the QXP, the station network Enhanced TM Plus (ETM+) and Terra ASTER (Advanced consisted of 60 primary stations, of which 35 are located in Spaceborne Thermal Emission and Reflection Radiometer) Qinghai Province and 25 in the Xizang (Tibet) Autonomous photos taken during 2000–02; glacier changes for the 384 Xiao and others: Observed changes of cryosphere in China

Table 2. Statistics of terminal changes of glaciers of western China based on 1950–70 topographic map and Landsat TM/ETM+ and Terra ASTER photos taken during 2000–02

Mountain Number of Number Speed of Number Number Speed of Period glaciers studied retreating terminal retreat vanished advancing terminal advance ma–1 ma–1

Qilian 257 230 –7.4 17 10 2.7 1956–2000 North slope of west Kunlun 37 30 –17 7 11 1970–2001 Muztage-Gongger 153 117 36 1963–2001 Karakoram 14 6 –32 8 25 1968–2002 A’nyeˆmaqeˆn 55 51 –11.5 4 6.4 1966–2000 Xinqinfeng-Malan 98 57 –4.5 41 3.6 1970–2000 Geladandong 44 38 –7.8 6 14.9 1969–2000 Puruogangri 10 7 –12.1 3 8 1974–2001 Gangrigabu, southeast Tibet 74 42 –13.8 32 19.9 1980–2001

764 glaciers in western China have been studied (Table 2). built for sea-ice observation, and ground-based radar The geometrically corrected error of the photos in different detection for sea-ice monitoring was established in the areas is usually smaller than two units (30–60 m for TM/ 1970s. From 1973, air photos and satellite images of sea ice ETM+, and 30 m for ASTER). The glaciers are not included in were used. Currently the sea-ice observations in the Bohai the statistics if the length changes are less than the errors. Sea and the northern Yellow Sea include (1) ground stations, (2) remote sensing, including air photos and satellite images, Data on frozen ground and (3) icebreaker observations. Ground surface temperature is a standardized monitoring River ice was observed at the national hydrological parameter at most meteorological stations. It is measured observation network governed by the Ministry of Water on the ground surface with half of the thermometer buried Resources of China (MWR). River-ice parameters such as the in the soil and half exposed to the air. Mean cold-season duration of river ice, freeze-up and break-up dates as well as temperatures and mean warm-season temperatures are the ice thickness were monitored at these stations. Here we average value of monthly temperatures from October to apply data from the Inner Mongolia section of the Yellow March and from April to September, respectively. SFG River, the Haarbin section of the Songhuajiang river and the depth is defined as the maximum frozen depth in each Fulaerji section of the Nengjiang river to identify river-ice cold season. changes during the past several decades in China. There are 119 meteorological stations in the QTP, of which 7 are located within permafrost regions with elevations higher than 4500 m and mean annual air OBSERVED CHANGES OF CRYOSPHERIC temperatures lower than –2.88C. Three of the seven stations COMPONENTS are free of permafrost (Maduo, Tuotuohe and Zhongxinzhan) Changes in glaciers due to the influences of high geothermal flux and/or rivers Mass-balance studies have been operated on several rela- (Zhou and Guo, 1982; Guo, 2000; Guo and others, 2000). tively easily accessible glaciers such as UG1, XDKMD, MK, Based on analysis of both mean daily and ground surface Qiyi and Hailuogou (Fig. 1). Between 1959 and 2002 the temperatures, there are no SFG thickness monitoring data at cumulative mass loss of UG1 equals a lowering of the 29 of the stations (20 of these are located in the south- glacier surface of up to 10.6 m. This loss of mass has western region of the QXP), either because of lack of SFG or accelerated since 1995. For instance, the cumulative loss of because the ground in this area normally freezes for only a mass between 1995/96 and 2001 accounts for 42% of the few nights during the winter. Freezing depths have been total loss since 1959. The mass balance at XDKMD has been monitored at 86 stations. These stations are distributed negative since 1993, and has continued to display an around the fringes of the QXP, while most of the inland areas accelerated loss of mass since then. Mass balance at MK are underlain by permafrost (Li and others, 1996). The data displays a parallel trend to that of XDKMD. Mass balance at used in this study are available from 1967 to 1997 at Qiyi from 1974/75 to 1977/78 was averaged to be 256 mm; 50 stations over the QXP after some minor adjustments and from 1984/85 to 1987/88 it was 4 mm, while in 2001/02 and estimations for missing data. SFG data in Xinjiang and the 2002/03 it was –861 and –360 mm, respectively. It is Hexi Corridor of Gansu Province are also derived from calculated that the rate of mass loss was 25.2 mm a–1 routine meteorological stations. Among the stations selected between 1976 and 1986, and –35.4 mm a–1 between 1986 for SFG studies are 10 in north Xinjiang, 9 in south Xinjiang and 2002. Hailuogou glacier, located at the east margin of and 17 in the Hexi Corridor. the Tibetan Plateau, has displayed a continuously negative mass balance since 1960. Sea- and river-ice data For the past few decades, terminal changes of China’s Observation stations surrounding the Bohai Sea were set up glaciers have been directly measured at several sites in the 1950s. The observing handbook of offshore sea ice considered to be logistically and morphologically ‘easy’. was produced in 1959, and routine observations maintained Terminal changes of UG1 have been observed since 1959 since then. Since 1969, three ice-breaking vessels have been based on a long-term monitoring program. Changes of other Xiao and others: Observed changes of cryosphere in China 385

Between the 1950s and 2000, these glaciers retreated >500 m (Fig. 2b). Although retreat is the overwhelming trend, some glaciers in the central and southeastern QXP are still advancing (Fig. 2c). This is probably due to the response of the glaciers to the present cooling of the local climate (Shi and others, 2006), or due to enhanced transport of moisture to high altitudes (Hewitt, 2005; Liu and others, 2006). Glacier changes for 764 glaciers in western China were studied (Table 2) using digital photos from Landsat TM/ETM+ and Terra ASTER. The changes have varied remarkably in different regions. In total, about 80.8% of the studied glaciers are retreating or have vanished, while 19.2% are advancing (Fig. 3). For each region (mountain range), the number of advancing glaciers is much less than that of retreating glaciers. The percentage of advancing glaciers on the margin of the QXP (e.g. Qilian, A’nyeˆmaqeˆn and Qomolangma) is higher than in the interior. Because of the resolution limits of the photos, changes of some small Fig. 1. Changes of mass balance of five glaciers in west China in glaciers and some stable glaciers cannot be determined; this recent decades. should be considered when assessing the overall changes of glaciers. Based on the above observations and the typical glacier monitoring, Yao and others (2004) estimated a shrinkage glaciers were based on field investigations by various 2 scientists in different years (Table 1; Fig. 2). of 3790 km of glacier areas in approximately the past 40 years, which equals a mean glacier thinning rate of Sporadic field investigations in the past few decades show –1 that changes of glacier lengths vary in different areas. The around 0.2 m a . breadth of these changes varies because of different climatic Changes in frozen ground conditions, glacier sizes and facings. Relatively small changes were found in the Qilian Shan and in the central Permafrost QXP and in some large glaciers in the Himalaya. We define Since 1964, a 35 m deep hole for measuring soil tempera- these changes of <500 m between the 1950s and around ture was measured at Fenghuoshan, central QXP. Ground 2000 as a small retreat (Fig. 2a). temperatures at different depths were read in 1964, 1984, Larger retreats are occurring on glaciers in the margins 1992 and 1996. Stable ground temperatures were observed of the QXP, the Pamirs and in the western Tian Shan. between 1964 and 1984, but after the mid-1980s turned into

Fig. 2. Changes of glacier terminus (length) based on ground investigations: glaciers that underwent small terminal retreats (a), large terminal retreats (b) and terminal advances (c). Note that the y-axis unit in (a) is 100 m, while in (b) and (c) it is 500 m. 386 Xiao and others: Observed changes of cryosphere in China

Fig. 3. Percentages of retreating glaciers among a total of 764 glaciers in China during the past five decades. Fig. 5. Ground temperature variation at 5 m depth below natural surface along Qinghai–Xizang highway since 1995. a warming trend for the surface soil within 20 m (Fig. 4) (Zhang, 2000). According to Zhao and others (2003), permafrost temperatures over the hinterland of the QXP depth played a minor role. The duration of SFG shortened by increased by about 0.2–0.58C between the 1970s and the >20 days from 1967 through 1997, mainly due to the earlier 1990s. Permafrost temperatures were measured at four sites onset of thaw in spring rather than the late onset of freeze in along the northern section of the Qinghai–Xizang highway: autumn. Chumaer river, Kekexili, Kunlun Pass and Fenghuoshan The average and maximum SFG depths and the duration (Fig. 5). At these sites, ground temperature at 5 m depth was of the frost period at 10 cm depth in Xinjiang during the averaged to have increased up to 0.58C over the period period 1961–2002 are analyzed by Q. Wang and others 1995–2002 (Wu and Liu, 2004; Zhao and others, 2004). (2005). The results show that the average and maximum Permafrost temperatures increased by about 0.2–0.48C from depths of the freezing front and the duration of the frost 1973 to 2002 at 16–20 m depths in the Tien Shan region period are decreasing due to warmer climate in Xinjiang in (Qiu and others, 2000; Zhao and others, 2004). Permafrost recent decades. Such changes have become more significant surface temperatures increased by about 0.7–1.58C over the since 1986, after an obvious shift of local climate towards a period 1978–91 from the valley bottom to the north-facing warmer, wetter regime. Significant decrease of the max- slopes in the Da Hinggan Ling in northeastern China (Zhou imum depths reached by the freezing front occurred in the and others, 1996). Permafrost temperature at the depth of the mid-1980s, both in south and north Xinjiang. zero annual temperature variation increased by about 2.18C Data on the upper and lower limits of SFG, as well as on on the valley bottom, 0.78C on the north-facing slopes and the duration of the frost period, collected from 17 meteoro- 0.88C on the south-facing slopes. The active layer increased logical stations in the Hexi Corridor in the period 1958– by about 40 cm at Jiagedaqi over permafrost, from an 2003 show a rapid decrease in the depth reached by the average of 240 cm in the 1960s to an average of 280 cm in freezing front in the mid-1980s. This coincides with an the 1990s (Liu and others, 2003). In the south-facing slope increase in winter temperature over the same period. The areas where no permafrost exists, soil temperatures at the minimum air temperature in winter controls the maximum lower limits of the SFG increased by about 2.48C (Zhou and depth reached by the freezing front and the duration of the others, 1996). frost period (Guo and others, 2005). In northeast China, there are few SFG studies, but since Seasonally frozen ground an obvious trend of increased air temperatures has been We discuss SFG changes over three major regions: the QXP, observed for the past 50 years, it is clear there has been a northwest China (including Xinjiang and the Hexi Corridor reduction of SFG. Observations show a rapid increase of of Gansu Province) and northeast China (Fig. 6). winter air temperature in northeast China in the past five Over the QXP, the SFG thickness decreased over a range decades (Sun and others, 2005), which is harmful to the 0.05–0.22 m from 1967 through 1997 (Zhao and others, preservation of SFG. 2004). The driving force for the decrease was the significant warming in cold seasons, while changes in snow-cover

Fig. 6. Changes in maximum frozen depths of the seasonally frozen Fig. 4. Ground-temperature changes in a hole at Fenghuoshan. ground over the QXP, Hexi Corridor and Xinjiang. Xiao and others: Observed changes of cryosphere in China 387

Fig. 7. Interannual variability of snow cover over China during the past 50 years: (a) Xinjiang, (b) QXP and (c) IM-NEC region.

Changes in snow cover Using scanning multichannel microwave radiometer (SMMR) data, covering 2500 cells of a 0.58 0.58 latitude–longitude grid between 1978 and 1987, Li (1999) showed the spatial pattern of average snow depth (cm) during the winter snow maxima (January and February) in west China. The spatial distribution of snow cover is very uneven geographically. Snow depths vary remarkably in altitude, especially between large mountains and basins in northwestern China. The highest snow depth is seen in the Altai Shan, followed by the Tien Shan, Pamirs, Karakoram and Kunlun Shan. Snow cover is also noted in the Elgis and Ili valleys. In contrast, snow cover is rare in the Tarim Fig. 8. Changes in sea-ice area over the Bohai Sea (a), north Yellow basin, Lop Nur and Badain Jaran desert. In the Junggar Sea (b) and the two regions combined (c). basin, snow is usually light, and frequently of short duration. Long distances to moisture sources, and the blocking effect of the mountains, keep the basins very dry. Snow cover, at about 59% in winter, is far from a pervasive however, a prolonged decrease in snow cover was seen, but feature over the QXP. Heavy snow cover is present only in not as great as the three previous snow-deficit periods. In the the peripheries, including the Himalaya, Pamirs, Nyain- 1960s and early 1970s, measured snow cover was lower qeˆntanglha and eastern Tanggula Shan. In the vast interior than at any other time in the second half of the 20th century. (e.g. Qaidam basin and Yarlung Zangbo valley), snow Over the QXP, long-term variability of snow cover is cover is rare, and in the northern QXP it is light and of characterized by large interannual variabilities superim- short duration. posed on a continuous increasing trend. Furthermore, the Annual variations of snow cover in the three major snow- annual amplitude of snow-cover variability has increased cover regions (Xinjiang, QXP and IM-NEC) were studied significantly since the 1980s (Fig. 7b). Both extremely based on station observations (Fig. 7). For the Xinjiang and heavy and light snow-cover years occurred more frequently. IM-NEC regions, the results of ground observation coincide The anomalies did not appear to be outside the range of with those derived from SMMR–Special Sensor Microwave/ natural variability. Imager (SSM/I) (Li, 2000; Konig and others, 2001), but for the In the IM-NEC region, there are several scattered areas QXP there are biases because of insufficient station data with deep snow cover where large interannual variabilities over the plateau. in snow depth were observed. These areas include the Da Figure 7a shows a time series of the annual and spring Hinggan, the Xiao Hinggan and Xilinguole. There are also ablation season (March and April), number of days with large seasonal variations in snow depth. The deepest snow snow cover, and annual cumulative daily snow-cover depth cover appears in mid-winter, when the largest interannual over northwest China from 1951 through 1997. It demon- snow depth variations also occur. The overall trend in the strates that long-term variability of snow cover is normally IM-NEC regions shows a very slight decrease (0.1 m a–1)in characterized by random oscillation. Snow cover fluctuated snow depth (Fig. 7c). around the mean, alternating between heavy and light snow The annual snow-cover variations depend both on the cover. Neither abrupt changes nor continuation of snow amount of winter snowfall and on the winter temperatures. minima from the late 1980s and early disappearance of Li (2000) established relationships between snow cover and spring snow cover were found. From the end of the 1980s, winter air temperatures and precipitation, respectively for 388 Xiao and others: Observed changes of cryosphere in China

Table 3. Decadal characteristics of river ice in Ningxia–Inner Mongolian section of Yellow River for the past five decades

Decade Duration Ice thickness Formation date Freeze date Break-up date days m

1950s Mean 115 0.72 15 Nov. 1 Dec. 25 Mar. Deviation +1 +0.07 –2 –1 +1 1960s Mean 114 0.72 17 Nov. 30 Nov. 23 Mar. Deviation 0 +0.07 0 –2 –1 1970s Mean 119 0.69 16 Nov. 29 Nov. 26 Mar. Deviation +5 +0.04 –1 –3 +2 1980s Mean 116 0.64 16 Nov. 1 Dec. 26 Mar. Deviation +2 –0.01 –1 –1 –2 1990s Mean 105 0.49 18 Nov. 7 Dec. 21 Mar. Deviation –9 –0.16 +1 +5 –3

Mean 114 0.65 17 Nov. 2 Dec. 24 Mar.

the Xinijing, QXP and IM-NEC regions. They are: are available. River ice of the Inner Mongolia section of the Yellow River has been observed at the Sanhuhekou Xinjiang: Sd ¼ 20:86P 150:39T 738:8 Shizuishan, Baotou, Zhaojunfeng and Toudaogui hydro- Sn ¼ 0:48P 4:51T þ 35:2 logical stations. River ice of the Songhuajiang river has been QXP: Sd ¼ 1:99P 14:21T 121:1 observed at the Haarbin hydrological station, and of the Nengjian river at Fulaerji station. IM-NEC: Sd ¼ 11:95P 43:2T 279: Changes in river ice can be described as changes in Sd and Sn are the annual daily cumulative snow depth data freeze-up and break-up dates, length of frost period as well (cm) and annual number of snow-cover days, respectively, as ice thickness. P is the amount of winter snowfall (mm) and T is the mean Over the Inner Mongolia section of the Yellow River, air temperature during the snow-cover season. freeze-up dates are 5 days later in the 1990s than the 1950s– The interannual snow-cover variations based on the 90s average, and the break-up dates in the 1990s are 3 days above functions coincide well with the observed results, earlier than average. The frost period duration and the with an error range within 10%. However, these functions maximum ice thickness in the 1990s are 9 days shorter and cannot properly reproduce long-term changes in snow 0.16 m thinner, respectively, than average (Table 3; Fig. 9). cover, probably due to the large errors in measuring solid At Haarbin, the freeze-up date is 1 day later, and the precipitation. break-up date 1 day earlier, in the 1990s than the 1950s–90s average. The frost period duration and the maximum ice Changes in sea ice thickness in the 1990s are respectively 4 days shorter and The Bohai Sea and the northern part of the Yellow Sea are 0.21 m thinner than average. located on the south edge of the Northern Hemisphere sea At Fulaerji, the freeze-up date is 6 days later, and the ice (378050–408550 N, 1178300–1258300 E). Since the early break-up date 2.5 days earlier, in the 1990s than the 1950s– 1950s, sea-ice coverage and thickness over the oceans were 90s average. The frost period duration and the maximum ice monitored for assessment and prediction of harbor disasters. thickness in the 1990s are respectively 10 days shorter and Sea-ice disasters are classified into five grades according to 0.06 m thinner than average. the sea-ice cover and thickness (Zhang, 1993). Heavy sea-ice In three study areas, the frost period and the maximum ice disasters occurred in 1956/57 and the winter of 1968/69, and thickness have decreased during the period since the late less heavy disasters in the winters of 1967/68 and 1976/77. 1970s, when global and regional climate warming is most During the past five decades, the Bohai Sea ice cover has prominent. During the period 1959–2002, annual mean air 8 –1 shown a declining trend, while there is no obvious trend temperature increased by 0.48 Ca , and winter air tem- 8 –1 over the north Yellow Sea area. For the whole area, however, perature increased by 0.72 Ca over northeast China (Sun a general decline in sea-ice coverage has been shown since and Yuan, 2005), where the Songhuajiang and Nengjiang late 1960s (Fig. 8). rivers are located. Zhang (1993) detected the relationships between sea-ice cover and climate, showing that the cycles of solar activities and El Nin˜o events correlate to the anomalous sea-ice SUMMARY conditions. The maxima and minima of solar activities, and In the past century, global warming has affected the climate the intensity and timing of the El Nin˜o events, play a very in China. The annual mean air temperature in China for the important role in the occurrence of severe ice conditions. past century coincides with that of the Northern Hemisphere (S. Wang and others, 2005). The global temperature has Changes in river ice increased by 0.6 0.28C since 1860 (Houghton and others, We discuss the changes in river ice over two key areas – 2001), while temperature in China increased by 0.588C. The Inner Mongolia and northeast China – where river-ice data warmest years have occurred since 1983 on a global scale, Xiao and others: Observed changes of cryosphere in China 389

Fig. 10. Spatial distribution of changes of cryospheric components in China for the past approximately five decades.

The duration of shortened SFG varied in different regions. Significant changes also occurred in the inland and northeastern regions of the QTP. The cold-season air temperature is the main factor controlling SFG change. The warming trends of ground surface temperatures are more significant than air temperature. The sea-ice coverage over the Bohai and Yellow Seas has deceased since the 1980s. River-ice duration and ice thickness are also decreasing in northern China.

ACKNOWLEDGEMENTS This study is supported by the Ministry of Science and Technology of China (MoST) (2007CB411500; Fig. 9. River-ice changes in north China since the 1950s: (a) freeze 2005DFA20930), the Climate Project of the China Me- date, (b) break-up date, (c) river ice duration and (d) maximum ice teorological Administration, and the Hundred Talents thickness. Project of the Chinese Academy of Sciences. We thank V. Lytle and T. Villinger from the CliC International Project Office, R. Barry and an anonymous reviewer for help with and the most obvious warming occurs in the winter. Since our English. 1985, there have been 20 warm winters in China (S. Wang and others, 2005), contributing heavily to the decay of the cryosphere (Fig. 10). Glaciers in various areas of west China have experienced REFERENCES different change in recent decades. Although in most areas Balling, R.C., Jr and C.D. Idso. 2002. Analysis of adjustments to the glaciers are either retreating or have vanished (80.8%), some United States Historical Climatology Network (USHCN) tem- glaciers are still advancing (19.2%). It is essential to study perature database. Geophys. Res. Lett., 29(10), 1387. (10.1029/ the mechanism of the advancing glaciers in order to predict 2002GL014825.) the behavior of glaciers over the QXP and other high Asian Che, T., X. Li, P. Mool and J. Xu. 2005. Monitoring glaciers and mountains. associated glacial lakes on the east slopes of Mount Xixabang- Snow cover in western China did not experience a ma from remote sensing images. J. Glaciol. Geocryol., 27(6), continual decrease during the great warming periods of the 801–805. [In Chinese.] 1980s and 1990s. It is interesting to note that no correlation Guo, D. 2000. Talik. In Zhou, Y., D. Guo, G. Qiu, G. Cheng was identified between temperature and precipitation in the and S. Li, eds. Geocryology in China. Beijing, Science Press, snow-cover season. The long-term variability of western 145–156. [In Chinese.] Guo, D., Y. Zhou, G. Qiu and S. Li. 2000. Southwestern China China snow cover is characterized by a large interannual geocryological area (Qinghai–Xizang Plateau). In Zhou, Y., variation superimposed on a small increasing trend. D. Guo, G. Qiu, G. Cheng and S. Li, eds. Geocryology in China. Frozen ground, including permafrost, is decaying both in Beijing, Science Press, 299–365. [In Chinese.] the QXP and in northeast China. The most significant Guo, H. and 7 others. 2005. Interdecadal variability of seasonal changes occurred in the regions with thickest SFG, i.e. the frozen ground depth and its climatic reasons in Hexi Corridor of inland QXP, then the northeastern and northwestern QXP. Gansu. J. Glaciol. Geocryol., 27(4), 503–508. [In Chinese.] 390 Xiao and others: Observed changes of cryosphere in China

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